Isn't AWS Aurora PostgreSQL a completely different (but compatible) product to PostgreSQL? If so it would make sense that its ability to handle many connections is just an implementation detail
https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide...
AWS has "only" replaced the storage layer.
For MySQL any of the big users like facebook & co. Are running (heavy) forked mysql versions with changes they need.